1. Field of the Invention
The present invention relates to a technique of controlling storage and read of data on a mass storage device, such as a storage device and a node device, which is accessed by a plurality of clients.
2. Description of the Related Art
A large scale computing system connecting with a plurality of client computers (Hereinafter referred to as a “client”) via a network is typically constructed in companies. Such a computer system requires a mass storage device, what is called a storage device, to provide a variety of data and programs used for operations on respective clients. The storage device, which has a high capacity hard disk, enables users of clients to store and read out data and programs in the allocated area for themselves or in the shared area. One typical protocol to access to the storage device is SCSI (Small Computer System Interface). Recently, another protocol called iSCSI has been applied to divide a SCSI command into IP packets to be transmitted via a network. The use of iSCSI attempts to provide an environment where a greater number of clients or clients located in a remote place can access to the storage device.
One typical method for utilizing the storage device is a remote boot, that is, start-up of the client. In the case of the remote boot, an operating system required to the start-up and a variety of files, such as a configuration file are pre-stored in an area corresponding to the individual user on the storage device. Each client is capable of start-up with reading of these files from the storage device via the network.
When executing the remote boot, each client is not required to incorporate a hard disk but it is sufficient to have a ROM for storing relatively smaller boot program to achieve the access to the storage device. Unifying the management of the data on the storage device, instead of storing the data in the hard disk of each client, prevents important data from being lost or leaked even if the client is damaged or stolen, and thereby enhances the security. This application also has practical advantage that allows users to start up any client connected on the network with a certain setup proper to the user. Details of the remote boot has been disclosed in, for example, JP1994-332716A, JP2001-75853A, and JP2000-259583A.
In some cases, start-up operations at about the same time by a large number of clients cause the concentration of accesses to a storage device, which results in delay of file supply to a client, and thus takes a long time for the start-up. Some examinations by the inventor of this application have shown that simultaneous start-up operations by about 10 clients require long time, even to the extent that users feel inconvenient. Those problems thus typically occur in a company at start time of working hours when the start-up operations of clients tend to concentrate on.
One available application includes a node device, equipped with a cache memory, between clients and the storage device to reduce accesses to the storage device. In this system, the cached data in the node device may be supplied to clients without accessing to the storage device, and thus reduces the number of accesses to the storage device. Such a system, however, still has a problem of the start-up delay because of the concentration of accesses to the node device when a plurality of clients are attempting to execute the start-up.
The above-mentioned problem is not limited to the case of the remote boot or the storage device and the node device but similar problem arises in the process of reading of the data with a diversity of memory devices capable of receiving accesses from a plurality of clients.